Re: key change for TCP-MD5

["Steven M. Bellovin" <smb () cs columbia edu>]

On Tue, 20 Jun 2006 17:06:27 -0400, Ross Callon <rcallon () juniper net>
wrote:

> At 12:12 PM 6/20/2006 -0700, Bora Akyol wrote:
>
> > The draft allows you to have a set of keys in your keychain and
> > the implementation tries all of them before declaring the segment
> > as invalid.
>
> DoS against routers is of course a major concern. Using
> encryption has the potential of making DoS worse, in the
> sense that the amount of processing that a bogus packet
> can cause is increased by the amount of processing
> needed to check the authentication. If the router needs to
> check multiple keys in the keychain before declaring the
> segment as invalid, then this multiplies the effect of the
> DOS by the number of keys that need to be checked.
You're quite right, and the next version of the draft contains the
following additional text in the Security Considerations:

     Having multiple keys makes CPU denial of service  
     attacks easier.  This suggests that keeping the 
     overlap period reasonably
     short is a good idea.  In 
     addition, the Generalized TTL Security Mechanism
     <xref target="RFC3682" />, if applicable to
     the local topology, can help.
     Note that there would almost never be more than
     two keys in existence at any one time in any event.


                --Steven M. Bellovin, http://www.cs.columbia.edu/~smb